Physicians who transfuse patients often want to know the effect of the infusion immediately. Yet drawing blood during or after a transfusion subjects the specimen to many variables and increases the risk of obtaining results that don't accurately reflect the patient's condition. When can a post-transfusion sample accurately reflect the benefit of the transfusion? What analytes are affected by transfusion? What is the best way to coordinate specimen collection with a transfusion? The answers to these questions are largely a function of what is being drawn and why.
Cell counts drawn during a transfusion provide little useful information except in trauma cases where the patient’s condition is changing rapidly and must be monitored frequently. Once all donor cells have been transfused, accurate cell counts may be obtained immediately after transfusion. However, waiting an hour post-transfusion may yield more accurate results because of the many variables that affect donor cell distribution.(1)
The main objective in collecting a post-transfusion specimen is to acquire a homogeneous sample of patient blood and donor cells. A multitude of variables determine homogeneity including the status of the patient’s heart, kidneys, and circulatory system, the patient’s pre-transfusion blood volume, and the age and volume of the transfused cells. The homogeneity of circulating blood does not occur in patients receiving blood due to an acute hemorrhage until all hemorrhaging has been stopped.
The levels of several chemistry analytes can be elevated for prolonged periods after transfusion. During storage of a donor unit, a significant percentage of red blood cells can hemolyze. Therefore, post-transfusion specimens can have elevated levels of plasma hemoglobin, potassium, ammonia, urea nitrogen, uric acid, vitamin B12, LD, and serum iron.(2,3,4) These levels can remain elevated up to 24 hours, depending on the patient’s kidney function and other variables. Laboratories testing post-transfusion specimens for these analytes should consider documenting the potential affect of the transfusion on these tests when releasing results.
The Clinical and Laboratory Standards Institute (CLSI) has not established criteria for drawing post-transfusion cell counts. However, its venipuncture standard (GP41-A7) details the proper procedure for drawing blood from a patient receiving IV fluids of any kind, including donor blood.(5) The standard states drawing above an active IV is rarely acceptable and should be considered with great caution. (5-7)
Draws from the same arm as an infusion are to be avoided if at all possible. When unavoidable due to difficulty in accessing other veins, CLSI suggests having the infusion temporarily shut off (2 minutes) prior to the puncture, tightening the tourniquet below the IV site and performing the puncture below the tourniquet. Draws from the same arm in which fluids are being infused must be documented to accompany test results, including what was being infused.
In summary, drawing samples during or immediately following a blood transfusion is critical to managing anemic patients. While the results have a limited validity, they provide critical information on the effects of the transfusion and drive lifesaving medical decisions. When delaying the draw for an hour or two after the transfusion(s) have completed does not threaten the responsible management of the patient, the results will be more accurate and be valid for a longer period of time post-collection.
1 Becan-McBride K, Eisenbrey A, Haraden L. Venipuncture after transfusion. Adv. Med Lab Prof. Q&A column. January 25, 1999:4.
2 Narayanan S. The preanalytic phase an important component of laboratory testing. Am J Clin Pathol 2000;113:429-452.
3 Mayhre B. Iron values after transfusion. Tips on specimen collection. MLO. Montvale, NJ. 1997.
4 Young D. Effects of Preanalytic Variables on Clinical Laboratory Tests. AACC Press; Washington, DC:2007.
5 CLSI. Collection of Diagnostic Venous Blood Specimens, Approved Standard, GP41-A7, Clinical and Laboratory Standards Institute. Wayne, PA. 2017.
6 Watson R, O’Kell R, Joyce J. Data regarding blood drawing sites in patients receiving intravenous fluids. Am J Clin Pathol. 1983;79(1):119-121.
7 Read D, Huberto V, Arkin C. Effect of drawing blood specimens proximal to an in-place but discontinued intravenous solution. Am J Clin Pathol. 1988;90(6):702-706.